A. Field of the Invention
The invention relates to the field of metal forming, and particularly to an improvement to the method of making sandwich structures by superplastic forming and diffusion bonding.
B. Description of Prior Art
It has been known for many years that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges. For example some titanium alloys, such as Ti-6Al-4V, have been observed to exhibit superplastic characteristics.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult or, in some instances, not possible. A significant breakthrough in superplastic forming was made by Hamilton, et al., disclosed in U.S. Pat. No. 3,934,441, "Controlled Environment Superplastic Forming," incorporated into this specification herewith by reference. Simplified, the process involves placing a worksheet or blank over a cavity in a chamber. The blank is heated to a temperature where it exhibits superplastic characteristics after which pressure is applied to the blank, causing it to stretch and form into the cavity.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause co-mingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding (SPF/DB) in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No. 3,927,817, "Method of Making Metallic Sandwich Structures," by Hamilton, et al., and is herein also incorporated by reference.
Basically, the Hamilton, et al. method for making metallic, sandwich structure involves fabricating the structures from a plurality of metal blank workpieces. One or more of the blanks are coated in selected areas not to be diffusion bonded. The blanks are positioned in a stacked relationship and placed in a die assembly. The stack is diffusion bonded together in the uncoated areas by the application of pressure, and at least one of the blanks is superplastically formed against one or more of the die surfaces forming the sandwich structure. The core configuration is determined by the location, size, and shape of the joined areas.
The coatings to prevent bonding are typically called stopoffs. Typical stopoffs are yttria (Y.sub.2 O.sub.3) and boron nitride in suitable binders. The method of preparing and applying yttria stopoff in the SPF/DB process is fully described in U.S. Pat. No. 4,220,276, "Method for Fabricating Superplastically Formed/Diffusion Bonded Structures," by E. D. Weisert, et al., and is herewith incorporated by reference. In summary the prior art methods have used silk screening techniques or templates. Silk screening is preferred because invariably when using templates some of the sprayed or painted stopoff "bleeds" beneath the template reducing the accuracy of the stopoff pattern. But silk screening techniques are not readily applicable to highly curved surfaces in particular tubular shapes.
Additionally while the stopoff coating thickness, as applied, is relatively thin, on the order of 0.004 inch, the coating does tend to create a gap between the workpieces. This can present problems if the stack compresses tubular shaped workpieces. Just sliding the tubes over one another can cause the stopoff to be scraped off. This would surely happen if close tolerances are held between the opposing principal surfaces of the tubes.
Therefore it is a primary object of this invention to provide an improved method of applying stopoff materials to prevent bonding between portions of the workpieces in the method of making sandwich structures by superplastic forming and diffusion bonding (SPF/DB).
Another object of this invention is to provide an improved method of applying stopoff materials to prevent bonding between the portions of workpieces not to be diffusion bonded together in the method of making sandwich structures by superplastic forming and diffusion bonding.
A further object of this invention is to provide a method of compensating for the increase in thickness of workpieces used in the method of making sandwich structures by superplastic or diffusion bonding caused by the application of stopoff material applied to the surfaces of the workpieces to prevent bonding at selected locations.